A. Field of the Invention
The present invention relates to a friction facing and a lockup clutch using the friction facing, and more particularly to those to be used in a torque convertor for performing a lockup slippage control.
B. Background of the Invention
In general, a torque convertor is composed of a front cover and three kinds of vane wheels (impeller, turbine and stator). When the impeller is rotated together with a front cover, working oil is circulated through vanes of the impeller and subsequently moves against vanes of the turbine and the stator. Rotation of the impeller is thereby transmitted to the turbine. Some torque convertors are provided with a lockup clutch that may mechanically couple the front cover and the turbine with each other thereby bypassing power transmission via rotation of the impeller.
The working oil for controlling the engagement and disengagement of the lockup clutch is introduced through a gap between the inner circumferential portion of the turbine and the front cover during the disengagement. The working oil is caused to pass between the front cover and the lockup clutch and to flow from a space between the outer circumferential portion of the impeller and the outer circumferential portion of the turbine to a space between the impeller and the turbine. The working oil is merged into the working oil for hydraulically connecting the impeller and the turbine with each other and is discharged from a gap between the inner circumferential portion of the turbine and the stator or a gap between the inner circumferential portion of the impeller and the stator. When the lockup clutch is in engagement, the working oil residing between the front cover and the lockup clutch is drained through the gap between the inner circumferential portion of the turbine and the front cover so that the lockup clutch is brought into pressing contact with the front cover by the oil-pressure balance.
Some lockup clutches include a vibration dampening spring to reduce the impact forces experienced upon engagement of the lockup clutch. In torque convertors having such a lockup clutch, lockup slippage control has also been added. The lockup slippage control is an additional control that, in circumstances where suppression of vibrations by means of the damper characteristics of the lockup clutch are insufficient, the vibrations are absorbed by slidingly moving the friction facings of the lockup clutch that are in contact with the front cover. In this control, the above-described oil-pressure balance is adjusted on the basis of an engine RPM, an output RPM and gear speed to thereby control the slippage condition. If this control is effected, even in a region in which a relatively low speed is attained and in which the conventional lockup clutch could not be operated due to the problems such as vibrations, it is possible to operate the lockup clutch. Then, if the operational range of the lockup clutch is thus broadened, the torque transmission efficiency is enhanced and a fuel consumption rate is improved in a vehicle that is provided with this torque convertor.
In the case where the lockup slippage control is effected in the above-described manner, there might be problems of sliding characteristics or durability of the friction facings. Namely, if the sliding characteristics (smoothness of slippage) of the friction facings are worse upon slippage, wavy vibrations are generated in the torque transmission to cause a vehicle shudder. Thus, the sliding characteristics become serious. The friction facings are heated due to the slippage, as a result of which the service life of the friction facings is shortened and the durability thereof may be adversely effected. To cope with these problems, conventionally, working oil is introduced by providing the friction facings with oil grooves or the like, so that the change in frictional coefficient of the frictional surfaces of the friction facings is reduced to thereby enhance the sliding characteristics. At the same time, the friction facings are cooled down by the oil to thereby enhance its durability.
On the other hand, since the friction facings are ring-like members. Accordingly, if these members are manufactured by cutting the raw or base materials, the productive yield becomes worse in that the materials are not efficiently use. There is typically substantial amounts of waste material. For this reason, there is a conventional approach that the friction facings are divided in the circumferential direction to enhance the yield.
In the case where the friction facings are composed of a plurality of divided segments in this manner, conventionally, the divided segments are not intimately arranged but gaps are provided between the segments in order to enhance the sliding characteristics and the durability. Thus, since the oil is introduced into the gaps to equally function as the above-described oil grooves formed in the friction facings to enhance the sliding characteristics and the durability of the friction facings.
However, only with the simple arrangement in which the divided segments are arranged with gaps, these gaps are in communication from the outer circumferential side to the inner circumferential side of the friction facings. Then, the working oil residing on the turbine side of the lockup clutch is caused to pass through the gaps to flow toward the front cover. As a result, the pressure of the oil, on the turbine side, of the lockup clutch to be controlled by the lockup slippage control is decreased, so that it is difficult to perform the fine lockup slippage control.